#include<stdio.h>
int main()
{
	double x,y;
	scanf("%lf",&x);
	if(x>=0&&x<5)
	{
		y=-x+2.5;; 
		printf("%0.3lf",y);
	}
	if(x>=5&&x<10)
	{
		y=2-1.5*(x-3)*(x-3);
		printf("%0.3lf",y);
	}
	if(x>=10&&x<20)
	{
		y=x/2-1.5;
		printf("%0.3lf",y);
	}
}
